360-degree videos, also known as immersive videos or spherical videos, are video recordings where a view in every direction is recorded at the same time, shot using an omnidirectional camera or a collection of cameras. During playback on normal flat display the viewer has control of the viewing direction like a panorama. It can also be played on a displays or projectors arranged in a cylinder or some part of a sphere.
360-degree video is typically recorded using either a special rig of multiple cameras, or using a dedicated camera that contains multiple camera lenses embedded into the device, and filming overlapping angles simultaneously. Through a method known as video stitching, this separate footage is merged into one spherical video piece, and the color and contrast of each shot is calibrated to be consistent with the others. This process is done either by the camera itself, or using specialized video editing software that can analyze common visuals and audio to synchronize and link the different camera feeds together. Generally, the only area that cannot be viewed is the view toward the camera support.
360-degree video is typically formatted in an equirectangular projection and is either monoscopic, with one image directed to both eyes, or stereoscopic, viewed as two distinct images directed individually to each eye for a 3D effect. Due to this projection and stitching, equirectangular video exhibits a lower quality in the middle of the image than at the top and bottom.
Specialized omnidirectional cameras and rigs have been developed for the purpose of filming 360-degree video (invented in 2013), including camera rigs, GoPro's Omni and Odyssey (which consist of multiple action cameras installed within a frame), and the All-In-One VR cameras such as Vuze Camera, Nokia OZO, Kandao Obsidian, There have also been handheld dual-lens cameras such as Ricoh Theta V, Samsung Gear 360, Garmin VIRB 360, and the Kogeto Dot 360—a panoramic camera lens accessory developed for the iPhone 4, 4S, and Samsung Galaxy Nexus.
360-degree videos are typically viewed via personal computers, mobile devices such as smartphones, or dedicated head-mounted displays. Users can pan around the video by clicking and dragging. On smartphones, internal sensors such as the gyroscope can also be used to pan the video based on the orientation of the device. Taking advantage of this behavior, stereoscope-style enclosures for smartphones (such as Google Cardboard viewers and the Samsung Gear VR) can be used to view 360-degree videos in an immersive format similar to virtual reality. The phone display is viewed through lenses contained within the enclosure, as opposed to virtual reality headsets that contain their own dedicated displays.
In March 2015, YouTube launched support for publishing and viewing 360-degree videos, with playback on its website and its Android mobile apps. Parent company Google also announced that it would collaborate with camera manufacturers to make it easier for creators to upload 360-degree content recorded with their products to YouTube. However, in 2017, Google and YouTube began to promote an alternative stereoscopic video format known as VR180, which is limited to a 180-degree field of view, but is promoted as being more accessible to produce than 360-degree video, and allowing more depth to be maintained by not subjecting the video to equirectangular projection 
Facebook (parent company of VR headset maker Oculus VR) followed suit by adding 360-degree video support in September 2015, and subsequently unveiled reference designs for its own 360-degree camera systems known as Facebook Surround 360. Facebook announced in March 2017 that more than 1 million 360-degree videos had been uploaded to Facebook to date. Vimeo also launched 360-degree video support in March 2017.
Google Cardboard, which is typically distributed in the form of do-it-yourself kits consisting of low-cost materials and components, has been credited with helping virtual reality become more readily available to the general public, and helping boost the adoption of 360-degree video by publishers, such as mainstream journalists and media brands.
The use of the term "virtual reality" to describe 360-degree video has been disputed, as VR typically refers to interactive experiences wherein the viewer's motions can be tracked to allow real-time interactions within a virtual environment, with orientation and position tracking. In 360-degree video, the locations of viewers are fixed, viewers are limited to the angles captured by the cameras, and cannot interact with the environment. The non-dynamic nature of video also means that rendering techniques cannot be used to reduce the risk of motion sickness.
Many different spherical image projections can be used depending on the application. The equirectengular projection is used most, but the Cube Map, Equi-Angular Cubemap (EAC) or Pyramid projections could deliver better quality or save bandwidth.
Spherical videos are frequently in curvilinear perspective with a fisheye effect. The heavy barrel distortion often requires rectilinear correction before applications in detection, tracking or navigation. However, some recent techniques developed navigation capabilities directly based on spherical videos.
HomCam was the first wearable system that enable real-time streaming of 360-degree video. The 360-degree perspective is captured by a helmet-based omnidirectional video system.
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